Automotive combustion engines are driven over a wide range of oil temperatures, revolution speeds and loads. Therefore, in order to improve the mileage, the lubricating oils are required to be good in terms of the friction characteristic under a wide range of service conditions. Accordingly, it has been difficult to cover a wide range of service conditions only by conventional lubricating oils in which a variety of additive agents, beginning with friction modifying agents, are combined. For example, when the oil temperatures are low, they have not carried out the formation reaction of sliding products sufficiently, or have worn to be unable to stay on the sliding surface so that they cannot lower the friction resistance stably.
On the other hand, from the aspect of sliding members, a method is disclosed in which inorganic composite films containing manganese phosphate or oxides of molybdenum or tungsten are formed by chemical conversion treatment. However, since the films are thin chemical conversion films whose thickness is a couple of micrometers, it is possible to secure the initial adaptability, but it has been difficult to maintain the lubricating characteristic over a long period of time because said thin films have been consumed. Moreover, a method has been taken in which solid lubricants are applied on the surface of sliding members, but, when solid lubricants are simply applied on the sliding surface, the solid lubricants are consumed so that they are removed from the sliding surface, and accordingly it has been difficult to maintain the lubricating characteristic over a long period of time.
In general, in machinery provided with sliding parts, lubricating oils have been used in order to make the operation smooth. Most of the lubricating parts of engines are put into fluidically lubricating states. However, in the top and bottom of dynamic valve systems or pistons, the wear inhibition has been generally given by the addition of zinc dithiophsophate or zinc dithiocarbamate.
In internal combustion engines, as the measure of friction loss reduction or mileage, lubricating oils have been used, in lubricating oils which a variety of additive agents, beginning with friction modifying agents, are combined as set forth in Japanese Examined Patent Publication (KOKOKU) No. 3-23,595 and the like, for example. Conventionally, in order to lower the friction coefficient of lubricating oils, the addition of organic molybdenum compounds, the combined compounding of organic molybdenum compounds and metallic cleaning agents (Japanese Examined Patent Publication (KOKOKU) No. 6-62,983, for instance), the combined compounding of organic molybdenum compounds and sulfuric compounds (Japanese Examined Patent Publication (KOKOKU) No. 5-83,599, for example) or the like has been known.
In the sliding between engine cams and shims, the lowered friction coefficient aiming at improving the mileage and the anti-wearability aiming at the durability have been required strongly. Conventionally, shims have been subjected to manganese phosphate treatments aiming at improving the initial adaptability in sliding. In Japanese Unexamined Patent Publication (KOKAI) No. 2001-73,167 and Japanese Unexamined Patent Publication (KOKAI) No. 9-209,189, there is disclosed a method in which inorganic composite films whose surface contains iron phosphate at least and further oxides of molybdenum and/or tungsten are formed on the surface of iron or steel sliding members by chemical conversion treatment.